1. Field of the Invention
The present invention relates to the preparation of cyclic carbonates having a 6-membered ring by transesterification of 1,3-propanediol compounds with carbonic acid esters and subsequent distillative depolymerization of the oligo- or polycarbonate initially formed, both stages being carried out in the presence of catalysts from the group consisting of Sn, Ti and Zr compounds and, if appropriate, the distillation residue of the depolymerization being at least partly recycled into the transesterification. The cyclic carbonates which can be prepared according to the invention can furthermore be stabilized against undesirable polymerization. The present invention therefore furthermore relates to cyclic carbonates thus stabilized, which have small amounts of acids of sulfur or phosphorus or esters or salts thereof and, when exposed to heat, show a significantly lower tendency toward polymerization than those which have not been stabilized.
2. Description of the Related Art
As is known, carbonates (I) having a six-membered ring can be obtained by transesterifying 1,3-propanediol compounds (II) with carbonic acid esters (III), the hydroxy compounds R.sup.3 OH being split off, and depolymerizing the resulting oligo- or polycarbonates under the conditions of a vacuum distillation to give (I), and separating off and isolating (I). Such processes are described, for example, in U.S. Pat. No. 4,501,905 and U.S. Pat. No. 4,440,937. Strongly alkaline catalysts from the group consisting of oxides, hydroxides, alcoholates, carboxylates and carbonates of alkali metals (preferably Na and K), aluminum, thallium or lead are employed here for the transesterification. During the depolymerization, depending on the conditions applied, such as pressure, temperature and reaction apparatus, and the amount and nature of the catalysts used, a greater or lesser amount of a distillation residue of about 25 to, in the most favorable case, 5% of the oligo- or polycarbonate employed is obtained, which must be disposed of and accordingly reduces the yield of (I).
It has now been found that some of the disadvantages can be avoided by another reaction procedure, in which both the stage of transesterification and the stage of depolymerization are carried out in the presence of Sn, Ti or Zr compounds. To improve the process further, all or at least some of the distillation residue from the depolymerization can be employed again in the transesterification of (II) with (III), the amount of (II) and (III) employed for a particular amount of (I) can thus be reduced, and the yield of (I), based on (II) and (III) employed, can be increased accordingly.
As is known, cyclic carbonates can be converted into polycarbonates in the presence of various catalysts. In this context, reference may be made to the following publications: U.S. Pat. No. 4,501,905, U.S. Pat. No. 4,568,755, EP 236 862, U.S. Pat. No. 4,707,539, EP-188 204 and U.S. Pat. No. 4,252,750. Although such polymerizations proceed far more slowly, they nevertheless also already proceed noticeably without an additional catalyst if the cyclic carbonates are exposed to heat over a relatively long period of time, for example are kept in the molten state. In itself, such a polymerization does not have an adverse effect if a melt is in any case to be fed to a polycarbonate synthesis. However, such a tendency toward polymerization manifests itself in a very troublesome manner if a cyclic carbonate, for example, is to be stored in the molten state over a relatively long period of time for polymer reactions other than self-polymerization then to be carried out. The polymer content formed then also participates in such other reactions, but impairs their result, reduces reproducibility and can even cause unusable products. Even if copolymerization of various dioxanones are to be carried out, prepolymerization of individual dioxanones impairs the result of a copolymerization in an unforeseeable manner and at any rate complicates the polymerization process.
A particularly pronounced tendency toward oligomerization and polymerization is shown by the cyclic carbonates of trimethylolalkanes, i.e. 5-methylol-dioxanones of the formula (VI) ##STR2## in which R.sup.1 denotes C.sub.1 -C.sub.6 -alkyl, cyclohexyl or C.sub.6 -C.sub.12 -aryl.
The carbonate of trimethylolpropane, from which interesting paint bases can be prepared, may be mentioned above all (cf. EP 600 417, EP 665 260 and EP 703 230).